This invention relates to a fuel supply control system for internal combustion engines, and more particularly to a system of this kind in which the amount of fuel to be supplied to an internal combustion engine, which is determined based on engine operating conditions, is increased or decreased depending on variations in the engine rotational speed when the engine is at idle to thereby stabilize the engine rotational speed of the engine at idle.
Conventionally, fuel supply control systems for internal combustion engines are proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 60-249645 and Japanese Provisional Patent Publication (Kokai) No. 61-277837, in which when the engine is at idle, the difference between a desired idling engine rotational speed (e.g. an average value of engine rotational speed values at idle) and an actual engine rotational speed is determined, and the amount of fuel to be increased or decreased is determined based on the determined difference, to thereby increase the amount of fuel to be supplied to the engine by the determined fuel amount when the engine rotational speed is below the desired idling engine rotational speed and hence increase the engine rotational speed, and on the other hand decrease the amount of fuel to be supplied to the engine by the determined amount when the engine rotational speed is above the desired idling engine rotational speed and hence decrease the engine rotational speed, whereby the idling engine rotational speed is stabilized.
More specifically, in the above proposed fuel supply control systems, the amount of fuel to be increased or decreased is obtained by multiplying the difference between the desired idling engine rotational speed and the actual engine rotational speed by a predetermined coefficient. Accordingly, as the difference increases, the amount of fuel to be increased or decreased is increased in proportion to the increased difference, so that the engine rotational speed approaches the desired idling engine rotational speed more rapidly. Further, by setting the predetermined coefficient at a relatively great value, i.e. by setting the feedback gain at a greater value, the engine rotational speed approaches the desired idling engine rotational speed further more rapidly.
In the meanwhile, it is widely known that, in an internal combustion engine, the responsiveness of the engine rotational speed to a change in the amount of fuel supplied to the engine depends on whether or not the engine is engaged with the driving system of a vehicle on which the engine is installed, such as a clutch and a transmission.
More specifically, in the case where the fuel supply is increased to increase the engine rotational speed, there is a time lag, which is peculiar to the feedback system, from the time point of increasing the fuel supply, at which the engine output starts to increase, to the time point of actual increase in the engine rotational speed. This time lag depends on the scale of the feedback system. When the engine is not engaged with the driving system of the vehicle, as in the case of stoppage of the vehicle, the scale of the feedback system is relatively small, i.e. the operation steps of the feedback system comprise a shorter sequence of increasing (or decreasing) the fuel supply - rise (fall) in the engine torque - increase (decrease) in the engine rotational speed, so that the time lag is relatively small. On the other hand, when the engine is engaged with the driving system of the vehicle, as in the case of the vehicle running at a low speed with the throttle valve fully closed, the scale of the feedback system is relatively large, i.e. the operation steps of the feedback system comprise an extended sequence of increasing (or decreasing) the fuel supply - rise (fall) in the engine torque - increase (decrease) in the engine rotational speed which is associated with increase (decrease) in the rotational speed of driving wheels caused by way of the driving system of the vehicle by the increased (decreased) engine torque, so that the time lag is relatively large. Therefore, if the abovedescribed feedback fuel supply control responsive to the difference between the desired idling engine rotational speed and the actual engine rotational speed is carried out when the feedback system is associated with rotation of driving wheels driven by way of the driving system by the engine, for example, rise in the engine rotational speed to be caused by increase in the fuel supply takes place only after the rotational speed of driving wheels, i.e. the vehicle speed, has increased through increase in the engine output torque. A similar difference in time lag in the control due to different scales of the feedback system to that stated above also occurs when the fuel supply is decreased to decrease the engine rotational speed.
However, in the above fuel supply control systems, the feedback gain in the fuel supply control is set at a relatively great value so that the engine rotational speed approaches the desired idling engine rotational speed more rapidly when the engine is not engaged with the driving system. Therefore, if this relatively great value of feedback gair is applied when the engine is engaged with the driving system, i.e. when the time lag in the feedback control is longer, the engine rotational speed control by relatively large fuel supply through correction of the fuel supply by the relatively large gain continues to be carried out for a longer period of time until the engine rotational speed is actually changed, which may result in hunting of the engine rotational speed.